Discharge-excited rare-gas halide (excimer) lasers are efficient sources of high power ultraviolet (UV) light for use in spectroscopic and photochemical applications and are commercially available. A xenon fluoride laser, for example, has received considerable attention for use as a UV source in UV photolithography in the microcircuit fabrication field. However, a major obstacle to operation of rare-gas halide lasers is rapid degradation of the laser gas mixture. In order to operate such laser continuously over a long period of time, it is necessary to periodically or continuously replace the gas mixture in the laser. While it is possible to consider discarding an argon mixture after it has gone through the laser, the high cost of krypton and xenon makes it necessary to reuse these gases.
One method of recovering the noble gases for reuse is to separate the gaseous constituents of the mixture by fractional distillation of the mixture at cryogenic temperatures in a batch process. The disadvantage with cryogenic fractional distillation is that it is expensive, inconvenient and labor intensive. Also, a large volume of rare-gas mixture is required if continuous operation of the laser is desired.
More convenient cold trap batch and continuous processes have also been proposed, which aim to trap the contaminants that condense at or above the cold trap temperature. Some of these cold trap processes pass the halogen, particularly when the halogen is fluorine, thereby in theory removing only the undesirable contaminants. Unfortunately, it seems that there are always some gaseous contaminants which have a condensation temperature below the cold trap temperature and are therefore not removed. These unremoved contaminants tend to build up until a cold trap purified mixture also becomes unusable without other purification. Furthermore, a cold trap purification process generally requires liquid nitrogen for the cooling, which is inconvenient to store and handle.
Another approach has been to purify the rare-gas halogen mixture by removing the contaminants from the mixture through chemical reaction. Unfortunately, the halogens are very reactive, so the halogen must be removed also, either with or preceding the chemical removal of the contaminants. After removal of the halogen and contaminants by chemical reaction, fresh halogen is then added back to the gas mixture before it gets to the laser.
In chemical purification processes of this type, titanium is generally used to remove both the fluorine and the contaminants. In an article by P. M. Johnson et al. entitled "A Closed-cycle Gas Recirculating System for Rare-gas Halide Excimer Lasers," Appl. Phys. Lett. 32, 291 (1978), a two-stage chemical purification process is described where fluorine is removed in a first stage with titanium heated to 300 degrees C. and other impurities are removed in a second stage with titanium heated to 850 degrees C. However, unlimited closed-cycle continuous operation is not possible even with this two-stage purification system because some contaminants are not removed and tend to build up until a rare-gas fluorine mixture purified with this two-stage scrubber is also unusable without other purification.
The following additional articles are representative of the state of the art in this field:
Christensen, "High-Repetition-Rate XeF Laser with Gas Recycling," Appl. Phys. Lett. 30, 483 (1977); PA0 Burlamacchi et al., "Long-Life Operation of an XeCl Excimer Laser," Appl. Phys. Lett. 34, 33 (1979); PA0 Gower et al., "Gas Composition and Lifetime Studies of Discharged Excited Rare-Gas Halide Lasers," IEEE J. Quantum Electron. QE-16, 231 (1980); PA0 Tennant, "Control of Contaminants in XeCl Lasers," Laser Focus (Oct. 1981); PA0 Kutschke et al., "Rare Gas Recovery Systems for Rare Gas Halide Lasers," Rev. Sci. Instrum. 52, 1655 (1981); PA0 Mandl et al., "Selective Removal of F.sub.2 Impurity from NF.sub.3 /Xe/Ne, XeF Laser Mixtures," Rev. Sci. Instrum. 53, 301 (1982); PA0 Kearsley et al., "Cryogenic Gas Purification and Lifetime Extension of ArF, KrF and XeF Laser Gas Mixtures," American Institute of Physics Topical Meeting on Excimer Lasers, Incline Village, Nev., USA (January 1983).